It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.
In the past, X-ray CT has been extensively used in radiography to investigate the anatomy and function of the lungs. CT uses computer processed X-rays to produce tomographic images (virtual ‘slices’) of specific areas of a lung. Digital geometry processing is used to generate a three-dimensional image of the inside of the lung from a series of two-dimensional radiographic images taken around a single axis of rotation.
X-ray slice data is generated using an X-ray source that rotates around the object in a circular shroud. X-ray sensors are positioned on the opposite side of the circle from the X-ray source. Early versions of the imaging machines operated by rotating the X-ray source and detectors around a stationary patient. Following each complete rotation, the patient would be moved axially and the next rotation carried out. Newer machines have been designed to allow continuous rotation of the X-ray source and detectors while the patient is slowly and smoothly slid through the X-ray circular shroud. These are called helical or spiral CT machines.
CT can be used for imaging most body structures. It is particularly useful for detecting both acute and chronic changes of structure inside the lungs. These changes may not be detectable using conventional two-dimensional X-ray imaging. A variety of techniques are used. For example, evaluation of chronic interstitial processes such as emphysema or fibrosis is carried out by taking thin slices of data and applying high spatial frequency reconstructions.
One of the principal limitations of these approaches is the need to image the lung while it is stationary in order to minimise blurring. In particular, CT has poor temporal resolution limiting its use for a dynamic lung test. CT scanning is typically performed at two different time intervals—upon breath hold at inspiration or expiration—usually minutes apart.
This has obvious drawbacks and limits the ability of CT to be used for dynamic lung function testing. Specifically, it produces a sampling of the lung and an interpolation is required to deduce lung motion between the two steady state conditions, but such methods assume that the motion follows a linear or defined path. They cannot produce continuous real-time images that would provide information regarding lung motion and be used to detect diseases that cause subtle changes in lung structure or function.
With the advent of fast CT scanning, it has been possible to develop methods called 4D-CT that capture images over time to depict movement of body organs, such as inflation and deflation of a lung. A typical 4D-CT scan involves 10 to 50 rotations around the patient, each coordinated with the table moving through the circular shroud. Fast scanning can be carried out at a rate of about 4 rotations/sec, with the detector taking about 1000 ‘snapshots’/rotation. Each ‘snapshot’ is carried out at one position (angle or ‘projection’) of the X-rays source. Typically between 40,000 to 200,000 images are collected in a 4D-CT.
For example, 4D-CT has been used for measurement of lung function, including expansion using traditional absorption based imaging, but has the drawback of delivering significant levels of radiation dose to the patient. Visualising, controlling and tracking patient specific respiratory motion is key to more precisely targeted irradiation treatment for disorders such as chest and abdominal cancers that move with the diaphragm. Motion of cancers causes problems with irradiation treatment because moving targets may appear with distorted shapes and in wrong locations on CT images. In order to compensate for this, larger irradiation fields and concomitant large radiation doses are used to ensure that the tumour is not missed.
Accordingly, respiratory gating may be combined with the 4D-CT for treating ‘moving’ tumours such as lung tumours. One approach is through the use of a small “box” placed on the patient's chest/upper abdomen and specialised cameras are used to monitor the motion of this box during respiration. This information is used to correlate the position of the lung tumour with specific phases of the respiratory cycle. At treatment, motion of the box allows the treatment beam to be turned on and off (gating) during specific phases of the breathing cycle. (Jiang S B et al, Int J Radiat Oncol Biol Phys 2008; 71:S103-7; Phys Med Biol 2008; 53:N315-27).
Use of CT has increased dramatically over the last two decades. An important issue within radiology today is how to reduce the radiation dose during CT scanning without compromising the image quality. In general, higher radiation doses result in higher-resolution images, while lower doses lead to increased image noise and blurred images. However, higher radiation doses increase adverse side effects, including the risk of radiation induced cancer.
In the past, attempts have been made to reduce exposure to ionizing radiation during a CT scan including:
1. new software technology to more effectively utilise the data recorded at the detector,
2. individualising the scanning and adjusting the radiation dose to the body type and body organ examined, and
3. evaluating the appropriateness of avoiding CT scanning in favour of another type of examination.
Notwithstanding these efforts, the relatively high radiation dose imposed on a patient by a CT scan (especially by a 4D-CT scan), is a key problem acting against the increased use of CT. In addition to CT, there are many other forms of imaging capable of dynamic imaging of dynamic biological processes, such as breathing.
The fluoroscope is ideally suited to dynamic imaging of the thorax and is used in a wide range of procedures such as barium swallow examinations, cardiac catheterization, arthrography, lumbar puncture, placement of intravenous (IV) catheters, intravenous pyelogram, hysterosalpingogram, and biopsies including bronchoscopy. Fluoroscopy may be used alone as a diagnostic procedure, or may be used in conjunction with other diagnostic or therapeutic media or procedures.
One specific application of the fluoroscope, and related medical imaging configurations, is in the acquisition of dynamic sequences for CTXV analysis. CTXV analysis is possible from acquisition sequences including (but not limited to): multiple images acquired from multiple perspectives (projections); multiple images acquired from a number of perspectives (projections) simultaneously; and, continuous acquisition with a moving perspective.
The work of Keall et al (US20140192952) attempts to reduce the clustering of projections that occurs when gating to a respiratory signal and using a constant rotation speed in 4DCBCT. To avoid this clustering of projections and achieve an even distribution of projections around the subject, Keall changes two variables: (1) the rotation speed of the gantry and (2) the time interval between projections.
With the advent of advanced imaging systems such as those used in CTXV and advances with 4D-CT technologies, fast imaging rates are achievable. These higher rates of image acquisition allow for more complex timing of when images are acquired throughout a dynamic event. This increased speed of acquisition allows for enhanced temporal resolution of rapidly occurring dynamic processes or events.
Accordingly there is an ongoing need to improve the quality of information derived from scanning without increasing, or preferably decreasing patient radiation exposure.
In the field of imaging based motion analysis, a key parameter that affects the image quality and dynamic range of the measurement is the magnitude of the sample's displacement that occurs between sequential images. This will be determined by the speed of the sample and the rate of acquisition. If the sample displacement is too large or too small, the measurement quality will be degraded. Imaging of samples that exhibit a large range of speeds during the measurement sequence using a constant acquisition rate will therefore inevitably cause degraded measurement quality for some of the frames.
In the field of imaging based motion analysis there are several techniques that are commonly employed to improve the quality of the motion measurement or the dynamic range of the measurements. One such example is the use of a ‘frame skip’ in a PIV (particle image velocimetry) analysis. This is where the image analysis compares images that are more than 1 image apart. As such the optimum signal to noise can be obtained for slower particles that have a very small displacement between sequential images.
When using X-ray images as the input, techniques such as these result in an increase in radiation dose imparted to the subject as extra (redundant) images are acquired. In X-ray imaging, extra images equate to extra dose delivered to the subject. Therefore for motion measurement from X-ray images, such as in a CTXV analysis, other techniques must be employed to overcome this.